For penguins, swimming beats flying so walking is okay

Penguins can move underwater with the speed of a swallow or swift, but cannot fly even as far as a chicken. How did a bird that in some cases shuffles 65km to its breeding grounds on unsuitable flippers lose its ability to fly there quickly?

A team of researchers from Britain, the United States, Canada and China have put forward a theory of how the penguin lost its ability to fly, published in the Proceedings of the National Academy of Sciences.

University of Aberdeen zoology professor John Speakman was part of the team and discovered that over time, penguins found it better to have a wing suited to swimming than flying.

He said: "Wings that have to do two jobs, flying and diving, can't be good at both. As a wing evolves to be better at diving it gets worse at flying, until the energy demands of flight become so great that eventually the penguin gives up flying altogether."

The albatross is an example of a long, light wing well-adapted for flying, while a short wing with heavy bones like that of the penguin is more suited to swimming.

"They are complete opposites," Speakman said.

As its wing evolved, the penguin would have become a better diver and a worse flyer, until one day the prospect of flying no longer appealed.

Dive-fishing auks are black and white like penguins, but airborne too.

To test the theory the team studied auks, a family of seabirds similar to penguins, that catch fish underwater but can still fly.

Speakman was able to work out how much carbon dioxide the birds produced as they swam, flew, or sat about.

From this, he could extrapolate how much energy these activities needed.

He explained: "We found auks have exceptional diving abilities, almost as good as a penguin, but their flight costs are enormous - the highest ever measured. This matched the theory exactly."

In their ability to fly and dive they were "on the cusp" of becoming like penguins, he said. The great auk, a species hunted to extinction in the 19th century, had already lost the ability to fly, in keeping with the theory.

Had there ever been a flying penguin?

"The fossil record doesn't contain a flying penguin," he said.

"The first one we have was already flightless, and that was about 60 million years ago."

So while it seems extremely mal-adaptive to see emperor penguins wobbling kilometres across the ice to their rookeries, it stems from a trade-off far back in the evolutionary chain in which their wings adapted to their increasingly aquatic environment and they lost the ability to fly.

"When you see them swimming underwater, you truly get a sense of what they gained in return," Speakman said.